Due to its characteristics of being easily applicable to various products and electrical characteristics such as a high energy density, a secondary battery is not only commonly applied to a portable device, but universally applied to an electric vehicle (EV) or a hybrid electric vehicle (HEV) propelled by an electric driving source. This secondary battery is gaining attention for its primary advantages of remarkably reducing the use of fossil fuels and not generating by-products from the use of energy, making it a new eco-friendly and energy efficient source of energy.
Recently, with the growing interests in smart grid, a large capacity energy storage device that stores excess energy is required to build an intelligent electrical grid. To build a large capacity energy storage device, a plurality of small capacity energy storage devices may be combined in series or parallel into a large capacity energy storage device.
As a small capacity energy storage device, a battery rack including a plurality of secondary battery cells is used. The secondary battery cells are electrically connected in series based on the required output voltage of the energy storage device, and the secondary battery cells are electrically connected to each other in parallel based on the required capacity of the energy storage device.
The large capacity energy storage device has a control unit to manage a plurality of battery racks included in the energy storage devices. The control unit measures the voltage of each battery rack, determines the state of charge of each battery rack, and using the voltage information, controls the charge and discharge of each battery rack. Thus, accuracy is important to a voltage sensor which measures the voltage of each battery rack. To this end, voltage sensors connected to each battery rack need to ensure accuracy through a calibration operation.
However, as disclosed in Korean Patent Application Publication No. 10-2011-0084754, when a plurality of battery racks is connected in parallel, and in this state, voltage sensors included in each battery rack measure the voltage, the voltage measured by the voltage sensors may be influenced by adjacent other battery rack. Thus, the voltage sensors connected to the battery racks may be calibrated by an inaccurate value.
Therefore, there is a need for a method for calibrating voltage sensors while avoiding the influence by adjacent battery racks.